Oral Contraceptive Use and Breast Cancer Risk According to Molecular Subtypes Status: A Systematic Review and Meta-Analysis of Case-Control Studies

Simple Summary Breast cancer (BrCa) is a heterogeneous disease with multiple intrinsic tumor subtypes evidenced by the joint expression of molecular tumor markers. Data from epidemiologic studies provide evidence supporting differential effects of oral contraceptives on risk of developing the distinct subtypes of breast cancer; while some studies suggest increased risk, others show its lack. Toward this objective, we conducted meta-analysis of case-control trials devoted to this topic. The results of our study suggest that the oral contraceptive use has different effects on the risk of developing the various molecular breast cancer subtypes. Abstract We conducted a systematic review and meta-analysis to investigate the effect of oral contraceptives (OCs) on risk of breast cancer (BrCa) by status of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). We searched the MEDLINE (PubMed), Embase and the Cochrane Library database and bibliographies of pertinent articles published up to 2020. Therein, we identified nineteen eligible case-control studies which provided data by breast cancer subtypes: ER-positive (ER+), ER-negative (ER−), HER2-positive (HER2+) and Triplet-negative (TN). Summary risk estimates (pooled OR [pOR]) and 95% confidence intervals (CIs) were calculated using fixed/random effects models. The summary meta-analysis showed that over-use of OCs led to significant increased risk of TNBrCa (OR = 1.37, 95% CI; 1.13 to 1.67, p = 0.002), as well as of ER−BrCa (OR = 1.20, 95% CI: 1.03 to 1.40, p = 0.019). There was also a significant reduction in the risk of ER+BrCa (OR = O.92, 95% CI: 0.86 to 0.99, p = 0.026,) and a slight reduction in the risk of HER2+BrCa (OR = 0.95, 95% CI; 0.79 to 1.14, p = 0.561) after taking OCs. Meta-analysis indicated that OC use has different impacts on risk of breast cancer subtypes defined by receptor status. The identified differences between individual subtypes of breast cancer may reflect different mechanisms of carcinogenesis.

studies suggest increased risk, others show its lack. Toward this objective, we conducted meta-analysis of case-control trials devoted to this topic.

Search Strategy and Selection Criteria
This systematic review with meta-analysis was designed according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [36,37] to determine if use of oral contraception, as compared to placebo, affects the risk of the breast cancer subtypes (Supplementary File S1-PRISMA 2020 Checklist). The bibliographic databases MEDLINE (PubMed), Embase and the Cochrane Library were searched for the identification of case-control studies that were conducted up to June 2020. The following search terms were used for all databases in various combinations: "oral contraceptives" or "birth control pill" AND "subtype breast cancer risk" or "ER+ subtype" or "ER− subtype" or "HER2 positive" or "TNBrCa". References of found articles, previous review articles and meta-analysis, and other relevant publications related to the topic of the work were also searched in order to identify further pertinent studies. Articles were initially evaluated according to title and/or abstract. Next, the decision was made to include or exclude after independent and double analysis, and full tests of selected studies. Relevant research data was extracted from the full-text works selected for inclusion.
We included publications written in English, based on case-control studies (populationand hospital-design), providing information about the association between oral contraceptive use and breast cancer by ER, PR or HER2 status, and the data contained therein were sufficient to calculate the odds ratio (OR) and the 95% confidence interval (CI). The exclusion criteria were as follows: insufficient data for calculating desired parameters, the results were reported as graphics; duplicate reports; and reviews or case only studies.

Data Extraction
The following data were extracted for each study: (i) clinical and methodological study characteristics such as last name of first author, publication year, country in origin, years of data collection, number of participants in case and control subgroups; (ii) information on use of OC in individual subgroups: whenever/never, duration, age at first use, and years since last use prior to diagnosis; (iii) in the original studies, different definitions and combinations of subgroup used; in our analysis, we grouped subtypes into the following four categories: ER-positive (regardless of their PR/HER2 status), ER-negative (regardless of their PR status), HER2-positive (absence of ER/PR), and Triplet-negative (absence of ER/PR/HER2).

Assessment of Study Quality
The Newcastle-Ottawa Scale (NOS) score was employed to evaluate methodological quality of included studies. With this tool, each study was assessed in three separate categories: selection of cases and controls, comparability of cases and controls on the basis of the design or analysis, and ascertainment of exposure. A maximum score was 9, of which 0-3, 4-6, and 7-9 scores were considered as low, fair, and high quality [38].

Statistical Analysis
The distribution of cases and controls at risk, ORs and 95% CI were separately identified by receptor status and for oral contraceptive use (ever or never) and by age of first use of OCs, duration of OC use, and years since last use of OCs prior to diagnosis, when data were available. We calculated the summary risk estimates and 95% CIs and plotted forest plots using random-effects models (DerSimonian-Laird method) for the association between ever oral contraceptives use and breast cancer by receptor status. The results indicated that the taking of OCs may have a high probability of increase in risk if OR was above 1, compared with non-use of OCs [39]. Heterogeneity among articles was estimated by engaging the I 2 statistic and p values associated with Q statistics. I 2 statistic indicates the percentage of total variability explained by heterogeneity, and values of ≤25%, '25%-75%', and ≥75% are arbitrarily considered as indicative of low, moderate, and high heterogeneity, respectively [40].
To explain the possible presence of publication bias, Begg's test (a rank correlation method based on Kendall's tau) and Egger's test (a linear regression method) were applied [41,42]. We also checked for funnel plot symmetry. Here, in the absence of bias, the plots will resemble a symmetrical funnel, as the results of minor studies will scatter at the left side of the plot and the spread will narrow among the major studies on the right side of the plot [43]. In order to explain the possible influence of covariates such as age at first OC use (<25 years/≥25 years), duration of OC taking (≥5 years/<5 years), and years since last OC use prior to diagnosis (<5/≥ 5) on risk of individual of breast cancer subtypes, we performed a meta-regression [44]. Meta-analysis of summary statistics from individual studies was performed through Statistica 13.3 software (StatSoft Poland, Kraków, Poland), using the Medical Package program.

Results
Searches of electronic databases allowed the identification of four hundred and fortythree (443) citations. Subsequently, three hundred forty-six (346) items were excluded on the basis of title and/or abstracts. In turn, ninety-seven (97) articles with potentially significant case-control works were identified and submitted for full-text assessment. Of these, seventy-eight (78) papers contained duplicate publications, or included insufficient data for calculating the desired parameters, or for other reasons, did not meet all the inclusion criteria. Finally, nineteen articles were included in the systematic review and meta-analysis. A detailed review of selection procedure is shown in Figure 1. The quality of the analyzed studies as assessed on the basis of the Newcastle-Ottawa Scale (NOS) ranged between 4-8, and the average score was 6.74 for included studies. Furthermore, 12 (63.16%) were considered high quality studies (NOS ≥ 7 points).
Thirteen (13)   The quality of the analyzed studies as assessed on the basis of the Newcastle-Ottawa Scale (NOS) ranged between 4-8, and the average score was 6.74 for included studies. Furthermore, 12 (63.16%) were considered high quality studies (NOS ≥ 7 points).
Thirteen (13) case-control studies were conducted in North, three (3) in Asia, and one each in Australia, and Europe, while one pooled study was conducted in the United States, Canada, Australia and Korea. The studies involved a total of 246,152 persons, including 31,250 cases of breast cancer and 214,902 people as control. Six studies exclusively included premenopausal women [45][46][47][48][49][50].
The present meta-analysis was conducted on the basis of data from nineteen casecontrol studies assessing the effect of oral contraceptives on the risk of individual molecular subtypes of BrCa. Characteristics of selected works are shown in Table 1.
In turn, a review of ten studies [46,48,49,51,52,54,57,[59][60][61] revealed that OC use longer than five years leads to non-significant decrease in ER+BrCa risk (OR = 0.91, 95% CI: 0.82 to 1.02, p = 0.124, I 2 = 82.41%); and use of OC for less than five years lowered the risk of cancer. This last result was statistically significant: OR = 0.93, 95% CI: 0.87 to 1.00, p = 0.042, I 2 = 53.09%. A comparison of OC use for more than five years and less than five years showed that the risk of developing ER+BrCa was similar: OR = 0.99, 95% CI: 0.91 to 1.08, p = 0.791. No evidence of publication bias was recorded for the duration of OC use, according to Begg's and Egger's tests ( Table 2). Meta-regression with the covariates of duration of OC use demonstrated lack of influence on ER+Ca subtype risk: β = 0.02, 95% CI: −0.12 to 0.15, p = 0.815.
We also assessed the relationship between ER+BrCa risk and years since last OCs use prior to diagnosis, based on five studies [46,48,52,54,60]. These revealed that the last use of OC in the period less than five years before diagnosis was associated with a non-significant reduction in subtype risk: OR = 0.94, 95% CI: 0.77 to 1.15, p = 0.555, I 2 = 66.34%. In turn, the last use of OC ≥ 5 years before diagnosis resulted in a marginal increase in cancer risk: OR = 1.05, 95% CI: 0.95 to 1.17, p = 0.346, I 2 = 49.58%. The Begg's and Egger's tests showed no publication bias for the last OCs use prior to diagnosis of this breast cancer, p = 0.693 and p = 0.798, respectively (Table 2). Moreover, multivariable meta-regression with covariate of years since last OCs use prior to diagnosis showed no significant influence on risk of ER+BrCa subtype: β = 0.03, 95% CI: −0.21 to 0.28, p = 0.798.
Finally, we examined whether the risk of developing ER+BrCa differs between premenopausal and postmenopausal women. It turned out that the risk was increased in premenopausal women, but the results were not statistically significant: OR = 1.07, 95% CI: 0.85 to 1.34, p = 0.575. No significant publication bias was detected by the Egger's test (p = 0.3752).
( Figure 3). The results of Begg's (p = 0.929) and Egger's (p = 0.927) tests indicate the lack of evidence of publication bias (  Beyond the aforementioned, six studies [47,50,51,55,58,59] rated the effect of duration of OC taking of OC on ER−BrCa subtype risk. OC use for a period ≥ 5 years showed a nonsignificant increase in risk: OR = 1.19, 95% CI: 0.81 to 1.76, p = 0.373, I 2 = 94.39%. In addition, OC self-administration for <5 years induced a non-significant risk of this cancer subtype developing: OR = 1.14, 95% CI: 0.93 to 1.40, p = 0.201, I 2 = 79.46. On the other hand, the comparison of these two groups showed a significantly higher risk of ER−BrCa in the case of OC use for more than five years: OR = 1.14, 95% CI: 1.01 to 1.27, p = 0.031. Results of Begg's test were inaccessible for variable ≥ 5 years, while that for the covariate < 5 years suggested the possibility of a publication bias (p = 0.041). In turn, Egger's test did not indicate publication bias for both covariates, ≥5 years (p = 0.230) and <5 years (p = 0.436). When comparing these groups, both the Begg's (p = 0.005) and Egger's tests (p = 0.000) showed the possibility of a publication bias; however, all publications in the funnel plot were placed inside the funnel (Table 2). Moreover, the results of multivariable metaregression for the covariates of the duration of OC use demonstrated that they did not have a significant effect on this subtype: β = −0.17, 95% CI: −0.46 to 0.11, z = −1.17, p = 0.241.
The meta-analysis of the risk of ER−BrCa subtype depending on the period of discontinuation taking of OC before diagnosis was based on four studies [45,47,51,59]. It showed a statistically significant increase for variable < 5 years (OR = 1.77, 95% CI: 1.35 to 2.32, p = 0.000, I 2 = 68.44%); and for the variable ≥ 5 years (OR = 1.41, 95% CI: 1.19 to 1.68, p = 0.000, I 2 = 56.77%). Beeg's test indicated no evidence of publication bias for variables of cessation of OC use <5 years, (p = 0.117) and ≥5 years (p = 0.117) before diagnosis. Data from Egger's test indicate publication bias for both covariates: < 5 years (p = 0.022) and ≥ 5 Beyond the aforementioned, six studies [48,51,52,56,59,60] rated the effect of duration of OC taking of OC on ER−BrCa subtype risk. OC use for a period ≥ 5 years showed a non-significant increase in risk: OR = 1.19, 95% CI: 0.81 to 1.76, p = 0.373, I 2 = 94.39%. In addition, OC self-administration for <5 years induced a non-significant risk of this cancer subtype developing: OR = 1.14, 95% CI: 0.93 to 1.40, p = 0.201, I 2 = 79.46. On the other hand, the comparison of these two groups showed a significantly higher risk of ER−BrCa in the case of OC use for more than five years: OR = 1.14, 95% CI: 1.01 to 1.27, p = 0.031. Results of Begg's test were inaccessible for variable ≥ 5 years, while that for the covariate < 5 years suggested the possibility of a publication bias (p = 0.041). In turn, Egger's test did not indicate publication bias for both covariates, ≥5 years (p = 0.230) and <5 years (p = 0.436). When comparing these groups, both the Begg's (p = 0.005) and Egger's tests (p = 0.000) showed the possibility of a publication bias; however, all publications in the funnel plot were placed inside the funnel (Table 2). Moreover, the results of multivariable meta-regression for the covariates of the duration of OC use demonstrated that they did not have a significant effect on this subtype: β = −0.17, 95% CI: −0.46 to 0.11, z = −1.17, p = 0.241.
In addition, the risk of developing ER−BrCa has been shown to be similar in premenopausal and postmenopausal women: OR = 1.05, 95% CI: 0.62 to 1.79, p = 0.850. No significant publication bias was detected by the Egger's test (p = 0.258).
Assessment of dependencies between risk of HER2+BrCa subtype and duration of OCs use was done on data from five studies [45,48,49,54,61]. Taking OC for over five years was associated with a slightly increased risk of cancer: OR = 1.09, 95% CI: 0.88 to 1.35, p = 0.447, I 2 = 19.01%. Results of the Egger's and Begg's tests did not reveal publication bias: p = 0.327 and p = 0.109, respectively. Moreover, the use of OC for less than five years was associated with a clear, albeit non-significant, reduction in the risk of HER+BrCa: OR = 0.88, 95% CI: 0.62 to 1.25, p = 0.480, I 2 = 58.34%. Both tests were insignificant for publication bias: Begg's test: p = 0.497, and Egger's test: p = 0.855. Additionally, the risk of developing HER2+BrCa was slightly increased when using OC for more than five years, but the results were not statistically significant: OR = 1.14, 95% CI: 0.84 to 1.54, p = 0.412. No evidence of publication bias was recorded for the duration of OC use, according to Begg's (p = 0.174) and Egger's tests (p = 0.605) (

Effects of Oral Contraceptive Use on HER2-Positive Breast Cancer
Analysis of relationship between OCs use and risk of HER2-positive BrCa (HER2+ BrCa) subtype included eight trials, and was based on data from 12,704 participants (cases: 1063, control: 11,641) [44,[47][48][49]53,54,57,60]. A statistically insignificant increase of cancer risk was reported in four studies [44,47,53,54], while in four trials, an insignificant risk reduction was seen [48,49,57,60]. The summary meta-analysis revealed that ever-use OC slightly decreased HER2+BrCa risk: OR = 0.95, 95% CI; 0.79 to 1.14, p = 0.561, I 2 = 26.62% (Figure 4). Both tests, Begg's and Egger's, indicated no publication bias: p = 0.138 and p = 0.252, respectively ( Table 2). Assessment of dependencies between risk of HER2+BrCa subtype and duration of OCs use was done on data from five studies [44,47,48,53,60]. Taking OC for over five years was associated with a slightly increased risk of cancer: OR = 1.09, 95% CI: 0.88 to 1.35, p = 0.447, I 2 = 19.01%. Results of the Egger's and Begg's tests did not reveal publication bias: p = 0.327 and p = 0.109, respectively. Moreover, the use of OC for less than five years was associated with a clear, albeit non-significant, reduction in the risk of HER+BrCa: OR = 0.88, 95% CI: 0.62 to 1.25, p = 0.480, I 2 = 58.34%. Both tests were insignificant for publication bias: Begg's test: p = 0.497, and Egger's test: p = 0.855. Additionally, the risk of developing HER2+BrCa was slightly increased when using OC for more than five years, but the Three studies evaluated dependence between period of last use of OC prior to diagnosis and occurrence of HER+BrCa subtype [45,48,54]. The period of less than five years was associated with a slightly higher, statistically insignificant risk of this subtype of breast cancer: OR = 1.09, 95% CI: 0.82 to 1.46, p = 0.555, I 2 = 0.00%. In turn, discontinuation of OC intake ≥ 5 years before diagnosis indicated a higher, but non-significant HER+BrCa subtype risk: OR = 1.12, 95% CI: 0.90 to 1.40, p = 0.295, I 2 = 0.00%. Results of Begg's test and Egger's test showed a lack of evidence of publication bias for the variable < 5 years: p = 0.602, and p = 0.485, respectively; the results for the variable ≥ 5 years also demonstrated absence of evidence of publication bias: p = 0.602 and p = 0.840, respectively. ( Table 2). The result of multivariable meta-regression for period of last use OC prior to diagnosis did not confirm the impact of these covariates on the BrCa subtype: β = 0.03 (95% CI: −0.33 to 0.39), z = −0.16, p = 0.8705.
Moreover, the risk of breast cancer was slightly lower, although not statistically significant, in premenopausal women: OR = 0.79, 95% CI: 0.58 to 1.06, p = 0.116. No significant publication bias was detected by the Begg's (p = 0.602) and Egger's tests (p = 0.781).
The relationship between the age of initiation oral contraception and the risk of developing a TNBrCa was investigated based on three studies [54,57,61]. Age less than 25 years was associated with a significantly increased risk of this cancer subtype: OR = 1.27, 95% CI: 1.08 to 1.50, p = 0.005, I 2 = 0.00%. Begg's test (p = 0.117) and Egger's test (p = 0.269) indicated no evidence of publication bias. In turn, starting of OC use at age ≥ 25 years was associated with slight increase in risk: OR = 1.03, 95% CI: 0.86 to 1.23, p = 0.758, I 2 = 0.00%. Begg's and Egger's tests demonstrated the lack of publication bias: p = 0.602 and p = 0.966, respectively (Table 2). Multivariable meta-regression for age at start of contraceptive pill self-administration did not confirm the impact of these covariates on BrCa subtype: β = 0.03 (95% CI: −0.33 to 0.39), z = −0.16, p = 0.871.
Cancers 2022, 14, x FOR PEER REVIEW 13 of 18 Figure 5. Forest plot and summary odds ratios on the association between risk of TNBrCa and everuse of oral contraceptives. Note: black diamonds represent the effect sizes; the horizontal lines denote the 95% confidence interval.
The relationship between the age of initiation oral contraception and the risk of developing a TNBrCa was investigated based on three studies [53,56,60]. Age less than 25 years was associated with a significantly increased risk of this cancer subtype: OR = 1.27, 95% CI: 1.08 to 1.50, p = 0.005, I 2 = 0.00%. Begg's test (p = 0.117) and Egger's test (p = 0.269) indicated no evidence of publication bias. In turn, starting of OC use at age ≥ 25 years was associated with slight increase in risk: OR = 1.03, 95% CI: 0.86 to 1.23, p = 0.758, I 2 = 0.00%. Begg's and Egger's tests demonstrated the lack of publication bias: p = 0.602 and p = 0.966, respectively (Table 2). Multivariable meta-regression for age at start of contraceptive pill self-administration did not confirm the impact of these covariates on BrCa subtype: β = 0.03 (95% CI: −0.33 to 0.39), z = −0.16, p = 0.871.
Based on the results of three studies, we performed an analysis of the influence of years from the last use of OC before diagnosis on the risk of TNBrCa [47,53,59]. The results of the meta-analysis showed a statistically significant increase in risk (OR = 1.60, 0.95 CI: Figure 5. Forest plot and summary odds ratios on the association between risk of TNBrCa and ever-use of oral contraceptives. Note: black diamonds represent the effect sizes; the horizontal lines denote the 95% confidence interval. Based on the results of three studies, we performed an analysis of the influence of years from the last use of OC before diagnosis on the risk of TNBrCa [48,54,60]. The results of the meta-analysis showed a statistically significant increase in risk (OR = 1.60, 0.95 CI: 1.01 to 2.53, p = 0.043, I 2 = 71.09%) for last use < 5 years. The results of Begg's test were inaccessible, while Egger's test revealed no evidence of publication bias (p = 0.581). An increase in the risk of the cancer subtype was also observed, but was statistically insignificant regarding last OC use ≥ 5 years before diagnosis (OR = 1.41, 95% CI: 0.93 to 2.15, p = 0.107, I 2 = 82.15%). Herein, the results of the Begg's test were unavailable, while the Egger's test indicated no evidence of publication bias (p = 0.248) ( Table 2). Multivariable meta-regression did not record the significant influence of covariates of period from the last use of OC before diagnosis on TNBrCa risk: β = −0.12, 95% CI: −0.74 to 0.50, z = −0.39, p = 0.696.
In addition, the risk of breast cancer was slightly lower, although not statistically sig-nificant, in premenopausal women: OR = 0.90, 95% CI: 0.67 to 1.21, p = 0.489. No significant publication bias was detected by the Egger's test (p = 0.279).

Discussion
Consistent with our results, the summary meta-analysis showed that ever-use of OC significantly increased the risk of TNBrCa, as well as of ER−BrCa. There was also a significant reduction in the risk of ER+BrCa and a slight reduction in the risk of HER2+BrCa after OC taking. This may indicate a protective OC effect in some molecular subtypes of BrCa. Furthermore, the initiation of OC use under the age of 25 years was associated with a significantly increased risk of TNBrCa and a non-significant reduction in the risk of ER+BrCa. In turn, the starting of OC use at the age of over 25 years resulted in a slight increase in the risk of TNBrCa and a slight reduction in the risk of ER+BrCa.
Duration of OC use longer than five years was found to lead to a significant increase in risk of TNBrCa, and to an insignificant increase in risk of ER−BrCa and a slightly increased risk of HER2+ OC. In contrast, taking OC for less than five years led to an increased, albeit insignificant, risk of TNBrCa, as well as risks of ER−BrCa. Our work demonstrated that a shorter period of OC use was associated with a statistically significant reduction in the risk of ER+BrCa, and also with a clear, albeit non-significant, reduction in the risk of HER2+BrCa.
The results of our study showed that the last use of OC in the period less than five years before diagnosis was associated with a statistically significant increase in risk of both TNBrCa and ER−BrCa, and with a slightly higher, statistically insignificant risk of HER2+BrCa. Moreover, we noted a non-significant reduction in risk of ER+BrCa. In turn, discontinuation of OC more than five years before diagnosis was associated with a statistically significant increase in ER−BrCa risk, and with a higher but non-significant risk of both TNBrCa and HER+BrCa, and a marginal increase in ER+BrCa. The only metaanalysis related to the above issue looked at the effect of oral hormonal contraception on the risk of TNBrCa. In this, Li et al. [64] showed that in women who took OC, there was a statistically significant increase in the incidence of TNBrCa: OR = 1.21, 95% CI: 1.01 to 1.41, p = 0.04.
In the previous article, we compiled, also using meta-analyzes, data from 79 casecontrol studies from 1960-2010. According to the results of the statistical analysis, there was an increased risk of BrCa with the use of OC before the first full-term pregnancy (OR = 1.14, 95% CI: 1.01 to 1.28, p = 0.036); as well as with the use of OC for more than five years (OR = 1.09, 95% CI: 1.01 to 1.18, p = 0.020). On the other hand, the use of OC before the age of 25 reduced the risk of BrCa: OR = 0.91, 95% CI: 0.83 to 1.00, p = 0.052 [34]. Our second meta-analysis included 42 studies published between 2009 and 2020. It turned out that the use of OC statistically significantly increased the risk of BrCa: OR = 1.15, 95% CI: 1.01 to 1.31, p = 0.036 [35].
The exact causes of the increased risk of ER-negative breast cancer with estrogen use have not yet been identified. The fact that estrogen promotes tumorogenesis of this subtype provides evidence that the presently observed effects occur via the influence of estrogens on the physiology of the tissues of the tumor-bearing host, rather than on the tumor cells themselves. One of the mechanisms by which the use of OC affects BrCa in women is the growth and angiogenesis of the tumor in the mammary gland caused by estrogen and ER positivity. A recent publication of Gupta et al. has proposed a second mechanism whereby estrogen promotes the growth of ER-negative breast cancer by systematically enhancing angiogenesis and stromal cell recruitment [65]. In the case of ER−BrCa and ER+BrCa, tumor growth may be favored by a mechanism that plays a major role in BrCa carcinogenesis and indicates that estrogen systematically increases vascular density and stromal cell recruitment [65]. The present observations indicate that estrogen increases the systemic capacity for angiogenesis, stromalization, and bone marrow cell recruitment, and that this mechanism is in part responsible for promoting tumorigenesis, including the growth of ER-negative tumors.
Great caution should be exercised in drawing final conclusions from our meta-analysis, as there were various limitations in conducting it. The value of the results may have been influenced by limiting the search results to works in English only. As a result of such a limitation, it was not possible to reach all the research related to the topic of our work. Secondly, retrospectively, self-reporting of use of oral contraceptives poses a risk of providing inaccurate data, and therefore the possibility of mistakes made in the recruitment to the control groups. Thirdly, there exists a possibility of errors occurring when cases are similar to the controls selected for the research, depending on the use of the OC. Fourthly, there is no available information on the type of oral contraceptive used, and also a possible source of bias is that the definition of "ever" for OC use is not unified, meaning that women are exposed to OC at various periods limited to the start and end of self-administration of pills. This may lead to misclassifications regarding the peak incidence of most cancers in old age with a long interval from last or first OC use, or the use of various hormone preparations in a woman's life.
Additionally, a sensitivity analysis was performed to assess whether excluding any of the studies would significantly affect the meta-analysis result. The exclusion of any study did not affect the results of the meta-analysis of HER2 positive and triple negative breast cancer subtypes. However, in the case of the ER-positive breast cancer subtype, the exclusion of the Cotterchio [51], Gaudet [55] or Work [59] trials changed the result of the meta-analysis to statistically insignificant. In addition, when analyzing the ERnegative breast cancer subtype, the sensitivity analysis showed that excluding Bethea [60], Cotterchio [51], Dolle [48] or Sweeney [52] would change the meta-analysis result to statistically insignificant. Despite this, the authors did not decide to exclude the above studies, as in more than half of the cases this would increase the value of the standard error.

Conclusions
The results of our study suggest that the use of oral contraceptives has different effects on the risk of developing the various molecular breast cancer subtypes; however, given that associations between them are still poorly understood, further research is required.